Biol Fertil Soils (2006) 42: 409–417 DOI 10.1007/s00374-006-0084-7
ORIGINA L PA PER
Fang-bai Li . Jun-jian Chen . Cheng-shuai Liu . Jun Dong . Tong-xu Liu
Effect of iron oxides and carboxylic acids on photochemical degradation of bisphenol A Received: 19 October 2005 / Revised: 16 January 2006 / Accepted: 16 January 2006 / Published online: 18 March 2006 # Springer-Verlag 2006
Abstract γ-FeOOH was initially prepared by hydrothermal process and then sintered at 280°C, 310°C, and 420°C, and four kinds of iron oxides were obtained and named as lepidocrocite, IO-280, IO-310, and IO-420. They were characterized by XRD, SEM, and BET techniques to disclose the crystal composition, morphology, and surface area. The XRD results show that IO-280 and IO-310 should consist of maghemite and hematite, while IO-420 should be pure hematite. With the increase of temperature, the specific surface area significantly decreased. To test the photocatalytic activity of iron oxides, bisphenol A (BPA) was selected as a model chemical. The results show that BPA photocatalytic degradation should depend strongly on pH value, light source, and the crystal structure of iron oxides and that IO-310 had the highest activity in the absence of oxalic acid under UV or visible light illumination. The dependence of BPA photodegradation on carboxylic acids in lepidocrocite-carboxylate systems was investigated. BPA degradation was promoted greatly by the addition of oxalic and citric acid, and slightly by tartaric, malonic, and malic acid. The first-order kinetic constant (k) of BPA degradation follows the order oxalic⋙citric≫tartaric>malonic>malic>without acid≈succinic acid. In iron oxide–oxalic acid systems, the reaction rate and efficiency of BPA degradation under UV light was much more than under visible light illumination, and the k value follows the F.-b. Li . J.-j. Chen . C.-s. Liu . T.-x. Liu Guangdong Key Laboratory of Agricultural Environment Pollution Integrated Control, Guangdong Institute of Eco-Environment and Soil Sciences, Guangzhou, 510650, People’s Republic of China F.-b. Li (*) South China Institute of Botany, Chinese Science of Academy, Guangzhou, 510650, People’s Republic of China e-mail:
[email protected] Tel.: +86-20-87024721 Fax: +86-20-87024123 J. Dong Zhangshan Institute, University of Electric Science and Technology of China, Zhongshan, Guangdong, 528402, People’s Republic of China
order: lepidocrocite>IO-280>IO-310>IO-420 under both UV and visible illumination. It is concluded that BPA photodegrdation should be dependent on the kind of carboxylic acid, iron oxides, and light sources in iron oxide–carboxylate system. The photochemical investigation of iron oxide–carboxylate complex systems is essential to the full understanding of the interactive mechanism of iron oxides and organic pollutants on the surface of soil in subtropical and tropical regions. Keywords Iron oxides . Bisphenol A . Photodegradation . Carboxylic acid . Oxalic acid
Introduction Geochemical processes at the mineral–water interface, including sorption, precipitation, dissolution, and electron transfer, markedly influence the fate, mobility, speciation, and bioavailability of inorganic and organic contaminants in the environment (Grundl and Sparks 1998). The investigation of the interfacial reaction mechanisms is critical for the full understanding of natural systems and the prediction of contaminants fate in the natural environment. It is noticeable that a heterogeneous catalytic process at mineral surfaces or geocatalysts should be the most important kind of geochemical process and is vital for organic pollutants degradation in natural environment (Schoonen et al. 1998; Rhoton et al. 2002; Swearingen et al. 2003). Therefore, it is important to understand the catalytic properties and activity of minerals (Huang 2000, 2004; Keppler et al. 2000). Iron oxides (including oxyhydroxides) are common in the earth’s crust and can function as geocatalysts (Cornell and Schwertmann 2003) because iron is very reactive in surface environments, and Fe(II) and Fe(III) can form stable compounds. The Fe released from minerals can be re-precipitated in the environment as various secondary oxide minerals, such as lepidocrocite (γ-FeOOH), goethite (α-FeOOH), akaganeite (β-FeOOH), feroxyhyte (δ′-FeOOH), ferrihydrite (Fe5HO8·4H2O), magnetite (Fe3O4), maghemite (γ-Fe2O3),
410
and hematite (α-Fe2O3). Most of iron oxides show semiconductor properties with narrow band gap (2.0– 2.3 eV) and should be photoactive under solar irradiation as photocatalysts or photosensitizers (Leland and Bard 1987). Photochemical transformations catalyzed by iron oxides have been investigated extensively (Kormann et al. 1989; Faust et al. 1989; Pal and Sharon 1998; Andreozzi et al. 2003; Fu et al. 2004). Cunningham et al. (1988) obtained the evidence of the photocatalytic formation of ·OH radical in illuminated suspensions of α-FeOOH. Unfortunately, the photochemical transformation rate for organic pollutants on the surface of iron oxides has been poorly accelerated and it might be negligible under visible light except for some certain organics, such as oxalate, sulfites. The photochemical transformation could be improved when iron oxides and polycarboxylic acids set up a photoFenton-like system (Zuo and Holgné 1992; Faust and Zepp 1993; Nadtochenko and Kiwi 1997; Voelker et al. 1997; Balmer and Sulzberger 1999; Mazellier and Sulzberger 2001; Bozzi et al. 2002) due to the formation of Fe(III)carboxylate complexes. The absorption of a photon results in an excited ligand-to-metal charge-transfer (LMCT) state of the complex. Polycarboxylic acids include low-molecular-weight (LMW) polycarboxylic acids (oxalic, malonic, malic, citric, tartaric, succinic acid) and dissolved organic matter. In fact, the photochemistry of Fe(III)-carboxylate complexes in natural aquatic environment, fog, precipitation, tropospheric aerosols, and soil solution has received considerable attention over the past three decades (Zuo and Holgné 1992; Faust and Zepp 1993; Nadtochenko and Kiwi 1997; Voelker et al. 1997; Balmer and Sulzberger 1999; Mazellier and Sulzberger 2001; Bozzi et al. 2002). During the photochemical reaction of Fe(III)-carboxylate complexes, dissolved Fe(II) and Fe(III) species, the superoxides and hydroperoxyl radicals (O2·−/·OOH), and hydrogen peroxides (H2O2) are formed as the key intermediates; the hydroxyl radical (·OH) is also produced. It may be hypothesized that organic pollutants will be attacked by hydroxyl radical and mineralized efficiently in above photochemical system. However, to our best knowledge, the effect of iron oxides and LMW polycarboxylic acids on the heterogeneous photodegradation of organic pollutants in the suspension has been poorly studied. In the present investigation, iron oxides were prepared and six kinds of organic acids including oxalic, citric, tartaric, malic, malonic, succinic acid were selected as carboxylic acids. Bisphenol A [2, 2-bis(4-hydroxyphenyl) propane] (abbreviated as BPA, CAS 80-05-07), as a representative compound among endocrines disruptors (EDCs), has been used extensively as a raw material for epoxy and polycarbonate resins and also as antioxidants in softeners, fungicides, and similar products (Cousins et al. 2002). The objectives are to investigate (1) the relationship between photocatalytic degradation of BPA and the properties of iron oxides; and (2) the effect of carboxylic acids and iron oxides on the photodegradation of BPA in the iron oxide–carboxylate complexes system.
Materials and methods The preparation of iron oxides First, lepidocrocite was prepared by using ferric chloride (FeCl2.4H2O), hexamethylenetetramine [(CH2)6N4], and sodium nitrite as the starting materials: 20 g of FeCl2. 4H2O, 28 g of (CH2)6N4, and 7.0 g of NaNO2 were dissolved in 400-, 80-, and 80-ml double-distilled water, respectively, and three solutions were obtained (Hall et al. 1995). Then a homogeneous mixture for the three solutions was prepared to obtain a bluish green precipitate. The precipitate remained in the suspensions was aged at 65°C for 3 h. The product was centrifuged at 3,600 r min−1 to separate precipitate. The precipitate was washed once with 95% alcohol, then three times with double-distilled water, and finally once with 95% alcohol to remove anions and organic impurities, and then dried at 338 K for 48 h. The dried gel was ground and an orange-colored lepidocrocite (γ-FeOOH) was obtained. Secondly, lepidocrocite powder was sintered at 280°C, 310°C, and 420°C for 2 h, respectively, and three kinds of iron oxides were obtained and named as IO-280, IO-310, IO-420, respectively. Characterization of iron oxides To determine the crystal phase composition of the prepared iron oxides, X-ray diffraction (XRD) measurement was carried out at room temperature using a Rigaku D/MAXIIIA diffractometer with CuKα radiation (λ=0.15418 nm). The accelerating voltage of 35 kV and emission current of 30 mA were used. The specific surface area, monolayer volume, micropore surface area, and total pore volume of all samples were measured by the Brunauer–Emmett– Teller (BET) method, in which the N2 adsorption at 77 K was applied and a Carlo Erba Sorptometer was used. The pore-size distribution (more than 2 nm) of the catalysts was determined by the Barrett–Joyner–Halenda (BJH) method. The surface morphology of the catalysts was observed using scanning electron microscopy (SEM Leica Stereoscan 400i series). BPA photodegradation experiment A Pyrex cylindrical photoreactor was used in the experiments to conduct photodegradation experiments, in which an 8-W LZC-UVA lamp (Luzchem Research) with a special emission peak at 365 nm was positioned at the center of the cylindrical vessel and used for photoreaction under UV irradiation. This cylindrical photoreactor was surrounded by a Pyrex circulating water jacket to control the temperature during the reaction. The photoreactor should be coated by aluminum foil to be away from indoor light irradiation before the addition of reaction solution. The reaction suspension was prepared by adding 0.25 g of iron oxide powder into 250 ml of aqueous BPA solution or the mixture solution of BPA and carboxylic acid. Before
411
H: hematite M: maghemite
1500
IO-420
H(113)
Relative Intensity (a. u.)
1800
H(012)
Fig. 1 The XRD graph of iron oxides powders
H(300)
Figure 1 showed the X-ray diffractograms (XRD) of iron oxides. Pure lepidocrocite powder was obtained first because all eight peaks (020), (120), (031), (111), (051), (220), (151), and (231) by XRD were attributable to lepidocrocite. IO-420 was pure hematite because its eight peaks (012), (104),
H(214)
Crystal structure
The specific surface area, micropore surface area, micropore pore volume, and total pore volume of iron oxides samples as listed in Table 1 were measured by the BET method. The BET data in Table 1 showed that the specific surface area decreased by increasing the sintering temperature. Lepidocrocite had the largest micropore surface area and micropore volume, while IO-310 had the largest total pore volume and the smallest micropore surface area and micropore volume. The specific surface area of lepidocrocite, IO-280, IO-310, and IO-420 was 115.44, 75.91, 60.48, and 29.40 m2 g−1, respectively, and their total pore volume was 0.2977, 0.3485, 0.3762, and 0.2747 m3 g−1, respectively. The morphology was examined by SEM, as shown in Fig. 2, indicating that the porous structure was attributed to the pores formed
H(116)
Results and discussion
Surface area and morphology
H(024)
The BPA remained during photoreaction was determined by liquid chromatography (Finnigan LCQ DUO). A Pinnacle II column (C18, 5 μm, 250×4.6 mm ID, Catalogue 9214575) was used for BPA analysis while an X Terra MS column (C18, 5 μm, 150×3.9 mm ID, Serial 186000478) was used for oxalic acid analysis. The mobile phase consisting of 70% HCN and 30% water was operated at a flow rate of 0.8 ml min−1, a maximum absorption wavelength of 278 nm was selected for BPA analysis. The concentration of total dissolved ferric ion (total Fe) was analyzed by AAS while that of Fe2+ was tested by colorimetrical ferrozine method (Balmer and Sulzberger 1999).
H(110)
Analytical method
(110), (113), (024), (116), (214), and (300) were attributable to hematite (Cornell and Schwertmann 2003). In the meantime, IO-280 and IO-310 should consist of the mixture of maghemite and hematite. The strongest XRD peak of hematite is the (104) peak with the dhkl-values of 2.69, while that of maghemite is the (311) peak with the dhkl-values of 2.518. However, the peak (104) with the dhkl-values of 2.51 for hematite and the peak (311) for maghemite were present at the same position of Bragg angles. The two peaks (110) and (311) of both IO-280 and IO-310 had the strongest value at 2θ=35.7°, whereas, the hematite (104) peak showed the second strongest value at 2θ=33.2°. It is difficult to determine the relative ratio of (110) peak for hematite and (311) peak for maghemite. Obviously, the content of hematite increased by the increasing temperature because it increased the relative intensity of (104) peak for hematite. It can be concluded that the phase transformation from lepidocrocite to maghemite and then to hematite occurred gradually with the increase of sintering temperature.
H(104)
the photoreaction, the suspension was magnetically stirred in a dark condition for 30 min to establish adsorption/ desorption equilibrium. The aqueous suspension containing BPA and iron oxide was irradiated by the UV/visible light lamp with constant aeration. At the given time intervals, the analytical samples were taken from the suspension and put in a dark box away from indoor light irradiation, and immediately centrifuged at 3,600 r min−1 for 20 min, then the supernatant was filtered through a 0.45-μm Millipore filter to remove the particles.
1200 H(110)+M(311) 900
IO-310
600
IO-280 300
Lepidocrocite (020)
(120)
(311) (111)
(051) (220) (151)
(231)
0 10
15
20
25
30
35
40
45
2 Theta (Degree)
50
55
60
65
70
412 Table 1 The physical and chemical properties of iron oxides Iron oxides
γ-FeOOH
IO-280
IO-310
IO-420
The specific surface 115.44 75.91 60.48 29.40 area (m2 g−1) Micropore surface 4.92 4.43 0.41 1.59 area (m2 g−1) Micropore volume 0.00062 0.00106 0.00000 0.00036 (m3 g−1) 0.2977 0.3485 0.3762 0.2747 Total pore volume (m3 g−1)
between iron oxides particles (Yu et al. 2003). SEM photos show the shape of iron oxides obviously changed from leaflike shape to nodule-like shape. The dependence of photocatalytic activity on iron oxides, light source, and pH value To test the photocatalytic activity of iron oxides under UV light, the experiments were carried out in the suspension
with an initial concentration of 8.66 mg l−1 BPA under an 8-W UV lamp (UVA) at pH 2.43, 4.56, and 11.45, as shown in Fig. 3a–c, respectively. The reaction lasted 4 h. Under UV light illumination, iron oxides were excited and a pair of electron and hole formed, as shown in Eq. 1. Then the excited electron was transferred to oxygen with formation of the hydroxyl radical as shown in Eq. 2. The photocatalytic degradation of BPA by hydroxyl radical was described as first-order kinetic model, which is expressed as k=−dC/dt. C is the BPA concentration and t is the reaction time. The first-order kinetic constant (k) could be calculated by k=−Ln(Ct/C0). Ct is the BPA concentration in any time during reaction while C0 is the equilibrium concentration of BPA before the reaction and after adsorption in the suspension. BPA could be degraded under UV light, and the rate of BPA degradation depended strongly on the pH value. At pH 4.56 (native pH value of solution), the k value for BPA degradation was 0.0020, 0.0039, 0.0045, and 0.0034 min−1 for lepidocrocite, IO-280, IO-310, and IO-420, respectively; it was 0.0015, 0.0013, 0.0015, and 0.0011 min−1 at pH 2.43 and 0.0039, 0.0139, 0.0197, and 0.0113 min−1 at pH 11.45, as shown in Fig. 4. The k value follows the order
A Lepidocrocite
B IO-280
C IO-310
D IO-420
Fig. 2 The SEM photograph of iron oxide powder
413 1.0
A pH 2.43
B pH 4.56
C pH 11.45
0.9 0.8
Ct/C0
0.7 0.6 0.5 0.4
Lepidocrocite IO-280 IO-310 IO-420
0.3 0.2 0.1 0.0 0
60
120
180
240 0
60
120
180
240 0
60
120
180
240
Reaction time (min) Fig. 3 The photocatalytic degradation of BPA under UV irradiation at pH 2.43 (a), pH 4.56 (b) and pH 11.45 (c), where Ct is the BPA concentration in any time during reaction while C0 is the equilibrium concentration of BPA before reaction (the same in Figs. 5, 6 and 8)
IO-310>IO-280>IO–420 >lepidocrocite at pH 4.56 and 11.45. The results show that BPA degradation should depend on the pH value of solution and the properties of iron oxides. In the strong acidic solution, the reaction rate of BPA degradation was much less than at pH 4.56 (native pH of BPA solution) and in strong alkaline solution, as shown in Fig. 3. The k value at pH 11.56 was two- to fourfold of that at native pH of BPA solution. The pH value can significantly affect the surface s chemistry of iron oxides. For example, the log Ka1 value þ þ of lepidocrocite Fe OH2 ¼ Fe OH þ H was 6.29 in 0.5 M NaClO4 solution (Zhang et al. 1992). In alkaline suspension (pH=11.45), the negative charges were distributed on the surface of lepidocrocite Fig. 4 The first-order kinetic constants on the different conditions
ð Fe OH ¼ Fe O þ Hþ Þ. This does not favor the adsorption of BPA. However, the quasi-Fermi level for electrons nEF* will decrease at 0.0591 V per pH unit as shown in Eq. 3 (Leland and Bard 1987). The decrease in the nEF* should promote the separation of electron-hole pairs with electron transfer to O2, and increased formation of hydroxyl radical formed as shown in Eq. 2, then resulted in a higher photocatalytic activity. On the contrary, in acidic suspension, the increase in pH value also leads to the increase in nEF*, but this will decrease the separation of electron-hole pair with a lower production of hydroxyl radical. Thus, the photocatalytic activity decreased even though lower pH values were favorable to adsorption of BPA. On the other hand, the
0.020
k (min-1)
0.016
Lepidocrocite IO-280 IO-310 IO-420
0.012
0.008
0.004
0.000 UV, pH 2.43
UV, pH 4.56
UV, pH 11.45
VIS, pH 4.56
414 1.0
Although the k value under VIS was almost the same as that under UV light for IO-280, IO-310, and IO-420, the quantum efficiency under UV light was much more than that under VIS because of the larger power of sodium lamp.
0.9
Ct /C0
0.8
(1)
O2 þ e ! O þ 2Hþ ! 2 OH 2
(2)
nF ¼ nF ðpH 0Þ 0:0591ðpHÞ
(3)
0.7
Lepidocrocite IO-280 IO-310 IO-420
0.6
0.5 0
30
60
90
120
150
Reaction time (min)
Fig. 5 The photocatalytic degradation of BPA under visible light (VIS) irradiation at pH 4.56
physical and chemical properties of iron oxides should be another factor in affecting their activity. A higher surface area of lepidocrocite had not led to a higher photocatalytic activity. Therefore, surface area should not be the decisive factor in determining the activity, whereas, the crystal structure might be more important. To test the photocatalytic activity of iron oxides under visible light, the experiments were carried out in the suspension with an initial concentration of 8.66 mg l−1 BPA under a 70-W sodium lamp (VIS) at pH 4.56, as shown in Fig. 5. The reaction lasted 2.5 h. The first-order kinetic constant (k) for BPA degradation under VIS was 0.0002, 0.0029, 0.0044, and 0.0034 min−1 for lepidocrocite, IO-280, IO-310, and IO-420, respectively, as shown in Fig. 4. The data indicate that the photocatalytic activity of lepidocrocite should almost be negligible, whereas, the IO310 had the highest photocatalytic activity under VIS illumination. The k value follows the order IO-310> IO-420>IO-280≫lepidocrocite under VIS illumination. 1.0
The dependence of BPA degradation on carboxylic acids A series of experiments were carried out to investigate the effect of carboxylic acid on BPA photodegradation at native pH value, as shown in Fig. 6. Lepidocrocite was used as catalyst. The initial concentration of BPA was 8.66 mg l−1 and that of all carboxylic acids was 0.8 mM. Fig. 6 shows that BPA photodegradation was greatly promoted by the addition of 0.8 mM oxalic and critic acid, and slightly promoted by addition of tartaric acid, malonic acid, and malic acid, and hardly affected by the addition of succinic acid. The first-order kinetic constants (k) were listed in Table 2. The k value was 0.0537, 0.0080, 0.0033, 0.0029, 0.0024 min−1 for the addition of oxalic, citric, tartaric, malonic, and malic acid, respectively. Obviously, the effect of oxalic acid was the most significant for the promotion of BPA degradation. The order of k value was oxalic⋙citric≫tartaric>malonic>malic>without acid. The concentration of total dissolved ferric ions (TotalFe) vs reaction time was showed in Fig. 7. Obviously, the concentration of total-Fe depended strongly on the kind of organic acids. As shown in Fig. 7, lepidocrocite was photodissolved significantly and the concentration of total-Fe was increased greatly in the first 5 min and then decreased on prolonging the reaction time when oxalic acid was added. By contrast, the concentration of total-Fe was Table 2 The first-order kinetic constants of BPA photodegradation in lepidocrocite–carboxylic acids solution
0.8
Ct /C0
Fe2 O3 ðγ FeOOHÞ þ hν ! e þ hþ
Carboxyl acids
0.6 Citric acid Succinic acid Malic acid Malonic acid Tartaric acid Oxalic acid Without acid
0.4
0.2
0
10
20
30
40
50
60
Reaction time (min)
Fig. 6 The BPA photodegradation in lepidocrocite–carboxylic acid solution with the initial concentration of 0.8 mM
Without acid Oxalic acid Citric acid Tartaric acid Malonic acid Malic acid Succinic acid
First-order kinetic constants, k (min−1) 0.0020 0.0537 0.0080 0.0033 0.0029 0.0024 0.0018
Relative coefficiency (R2)
0.9606 0.9611 0.9951 0.9915 0.9053 0.9858 0.9469
415 20
Oxalic acid Citric acid Tartaric acid Malonic acid Malic acid Without acid
Total-Fe (mg l-1)
15
10
5
0 0
10
20
30
40
50
60
Reaction time (min)
Fig. 7 The concentration of total Fe vs reaction time in lepidocrocite–carboxylic acid system
increased gradually on prolonging the reaction time for citric, tartaric, malonic and malic acid. Iron oxides and polycarboxylate group can form Fe(III)carboxylate complex on the surface or in the solution. Absorption of a photon by an Fe(III)-carboxylate speicies initiates the formation of short-lived intermediates that ultimately yield Fe(II) and a free carboxylate radical. Fe(II) would be quickly oxidized into Fe(III) in the presence of oxygen (under air-saturated condition). The dominant intermediates included the hydroperoxyl ·n−radical OOH O 2 , Fe(II), carboxylate radical (RCOO ), and the carbon-centered radical (>C·) (Zuo and Holgné 1992; Mazellier and Sulzberger 2001). For example, in the presence of oxalic acid, the Fe(III)-oxalate complex was formed on the surface of lepidocrocite, as shown in Eq. 4. When the Fe(III)-oxalate complex was excited, both Fe(II)
and oxalate radical were generated as shown in Eqs. 5 and 6. Oxygen trapped the electron from oxalate radical and hydroperoxyl radical OOH O formed, as shown in 2 Eqs. 7and 8 (Faust and Zepp 1993). Fe(II) reacted with OOH O 2 to form H2O2 as shown in Eq. 10 and Fe(III) reacted with OOH O 2 to form O2 and Fe(II), as shown in Eq. 10. Under the acidic pH range, the reaction with Fe(III) was very slow and the ·OOH radical would react with Fe(II) to produce H2O2. However, Fe(II) was re-oxidized to Fe (III) in the presence of O2 as shown in Eqs. 9 and 11, and BPA was oxidized by hydroxyl radical and mineralized step by step as shown in Eq. 12. Fe(III)-oxalate complex had a higher photochemical activity than the other Fe(III)carboxylate complex. It may be hypothesized that bicarboxylic acid with a shorter carbon chain in the presence of iron oxides might easily form a more stable Fe(III)carboxylate complex with a higher photochemical activity than that with a longer carbon chain. FeOOH þ H2 C2 O4 ! ½ FeðIIIÞðC2 O4 Þþ þ H2 O ½ FeðIIIÞðC2 O4 Þþ þ hν ! ½ FeðIIÞðC2 O4 Þ
(5)
½ FeðIIÞðC2 O4 Þ ! FeðIIÞ þ CO2 þ CO 2
(6)
CO 2 þ O2 ! CO2 þ O2
(7)
þ O 2 þ H ! OOH
(8)
1.0
A UV
1.0
0.8
0.8
0.6
0.6
0.4
0.4
B VIS
Ct /C0
Fig. 8 The photodegradation of BPA under UV (a) and VIS (b) in iron oxides–oxalic acid solution with an initial concentration of 0.8 mM
(4)
Lepidocrocite IO-280 IO-310 IO-420
0.2
0.2
0.0
0.0 0
10
20
30
Reaction time (min)
40
0
10
20
30
Reaction time (min)
40
416 Table 3 The first-order kinetic constants (k) and relative coefficiency (R2) of BPA degradation in iron oxides–oxalic acid solutiona Iron oxides
UV light k (min−1)
Lepidocrocite IO-280 IO-310 IO-420
Visible light R2
0.0483 0.0423 0.0374 0.0244
k (min−1)
R2
0.9948 0.9870 0.9825 0.9835
0.0163 0.9957 0.0127 0.9971 0.0091 0.9899 0.0051 0.9958 a 0 The initial concentration of oxalic acid COX is 0.8 mM, the initial concentration of BPA C0BPA is 16.78 mg l−1 under UV light and 16.71 mg l−1 under visible light
OOH þ Hþ þ FeðIIÞ ! FeðIIIÞ þ H2 O2 O 2
(9)
O 2 OOH þ FeðIIIÞ ! FeðIIÞ þ O2
(10)
FeðIIÞ þ H2 O2 ! FeðIIIÞ þ OH þ OH
(11)
OH þ BPA !!! ::: ! CO2 þ H2 O
(12)
BPA photodegradation in iron oxides–oxalic acid systems The effect of iron oxides on BPA photodegradation in aerated iron oxides (lepidocrocite, IO-280, IO-310, and IO-420) suspensions with the dosage of 1.0 g l−1 in the presence of oxalic acid without pH adjustment was shown in Fig. 8a and b. In the former figure, the initial concentration of BPA was 16.78 mg l−1 and photodegradation was achieved under UV light irradiation, whereas, the initial concentration of BPA was Fig. 9 The concentration of total Fe vs reaction time under UV (a) and VIS (b) in iron oxides–oxalic acid system with an initial concentration of 0.8 mM
16.71 mg l−1 and the photodegradation was achieved under visible light irradiation. In both cases, the initial concentration of oxalic acid was 0.8 mM. The reaction lasted 40 min. BPA degradation strongly depended on the kind of iron oxides and was described by the first-order kinetic model. The dependence of first-order kinetic constant (k) on iron oxides was listed in Table 3. Under UV light illumination, the k value was 0.0483, 0.0423, 0.0374, 0.0244 min−1, while it was 0.0168, 0.0129, 0.0093, and 0.0052 min−1 under visible light illumination for lepidocrocite, IO-280, IO-310, and IO-420, respectively. Obviously, the k value in the presence of 0.8 mM oxalic acid was higher than in the absence of oxalic acid as shown in Fig. 4. That indicates that BPA should be degraded much more efficiently with than without oxalic acid. The lepidocrocite–oxalic acid system showed the highest photochemical efficiency under UV and visible light. On the other hand, iron oxides–oxalic acid system had a much higher efficiency under UV light than under visible light. It was noticeable that the blue-greenish chromosphere was present during the reaction under both UV light and visible light. The intensity of color increased by increasing the initial concentration of oxalic acid. Siffert and Sulzberger (1991) reported that the chromosphere should be Fe(III)-oxalate surface complex. Balmer and Sulzberger (1999) reported Fe(III) species were mainly present as FeðC2 O4 Þ and FeðC2 O4 Þ3 in the Fe(III)-oxalate 2 3 system when the concentration of oxalate was higher than 3 0.18 mM. Both FeðC2 O4 Þ are much 2 and FeðC2 O4 Þ3 more efficiently photolyzed and the formation of Fe(II) and H2O2 is faster at high than at low oxalate concentration. In our study, the concentration of dissolved ferric ions (total Fe) by AAS depended on iron oxides, as shown in Fig. 9. Obviously, lepidocrocite was more easily dissolved than IO-420. Under UV light illumination, total Fe increased dramatically in the first 5 min and then dropped quickly because the concentration of oxalic acid decreased with reaction time. The higher concentration of dissolved
18
18
A UV
Total-Fe (mg l-1)
15
B VIS
15
Lepidocrocite IO-280 IO-310 IO-420
12
12
9
9
6
6
3
3
0
0 0
10
20
30
40
Reaction time (min)
50
60
0
10
20
30
Reaction time (min)
40
417
Fe(III) could increase the concentration of FeðC2 O4 Þ 2 and 3 FeðC2 O4 Þ3 in the solution with higher photochemical reaction rate. Under visible light illumination, total Fe was greatly increased in the first 5 min and then decreased gradually for lepidocrocite, IO-280, and IO-310, whereas, it was always increasing for IO-420 with prolonged reaction time, as shown in Fig. 9b. Siffert and Sulzberger (1991) reported that the rate of photochemical reductive dissolution of hematite was strongly wavelength-dependent and the photocatalytic degradation of oxalate in aerated hematite also depended on the incident wavelength, as the degradation rate of oxalate dropped above 390 nm (Siffert and Sulzberger 1991). In this investigation, the wavelength of sodium lamp emission was usually higher than 400 nm. Iron oxides could be significantly photo-dissolved with increase in total Fe, as shown in Fig. 9b. In fact, oxalate was photodegraded at a slower rate under visible light than under UV light, and the higher concentration of oxalate under visible light might lead to a higher concentration of total Fe during the later stage of reaction.
Conclusion Photocatalytic degradation of BPA on the surface of iron oxides depended strongly on pH value, light source, and the crystal structure of iron oxides. BPA degradation was promoted greatly by the addition of oxalic and citric acid, and slightly by the addition of tartaric, malonic, and malic acid. Iron oxide–oxalate system was the most photoactive. BPA photodegrdation was dependent on the kind of carboxylic acid, iron oxides, and light sources in iron oxide–carboxylate complex system, and the rate and efficiency of BPA degradation under UV light was higher than under visible light. Acknowledgements The authors wish to thank the project (No. 20377011) funded by China National Natural Science Foundation and the key project (No. 036533) funded by Guangdong Natural Science Foundation.
References Andreozzi R, Caprio V, Marotta R (2003) Iron(III) (hydro)oxidemediated photooxidation of 2-aminophenol in aqueous solution: a kinetic study. Water Res 37:3682–3688 Balmer ME, Sulzberger B (1999) Atrazine degradation in irradiated iron/oxalate systems: effects of pH and oxalate. Environ Sci Technol 33:2418-2424 Bozzi A, Yuranova T, Mielczarski J, Lopez A, Kiwi J (2002) Abatement of oxalates catalyzed by Fe-silica structured surfaces via cycle carboxylate intermediated in photo-Fenton reactions. Chem Commun 19:2202–2203 Cornell RM, Schwertmann U (2003) The iron oxides: structure, properties, reactions, occurrences and uses. Wiley-VCH, New York Cousins IT, Staples CA, Klecka GM, Mackay D (2002) A multimedia assessment of the environmental fate of bisphenol A. Human Ecol Risk Assess 8:1107–1135
Cunningham KM, Goldberg MC, Weiner ER (1988) Mechanisms for aqueous photolysis of adsorbed benzoate, oxalate, and succinate on iron oxyhydroxide (goethite) surfaces. Environ Sci Technol 22:1090–1097 Faust BC, Zepp RG (1993) Photochemistry of aqueous iron(III)polycarboxylate complexes: roles in the chemistry of atmospheric and surface waters. Environ Sci Technol 27:2517–2522 Faust BC, Hoffmann MR, Bahnemann DW (1989) Photocatalytic oxidation of sulfur dioxide in aqueous suspensions of α-Fe2O3. J Phys Chem 93:6371–6381 Fu HB, Quan X, Liu ZY, Chen S (2004) Photoinduced transformation of γ-HCH in the presence of dissolved organic matter and enhanced photoreactive activity of humate-coated α-Fe2O3. Langmuir 20:4867–4873 Grundl TJ, Sparks DL (1998) Kinetics and mechanisms of reactions at the mineral–water interface: an overview. In: Sparks DL, Grundl TJ (eds) Mineral–water interfacial reactions: kinetics and mechamisms. American Chemical Society, Washington, District of Columbia, pp 2–13 Hall PG, Clarke NS, Maynard SCP (1995) Inelastic neutron scattering (TFXA) study of hydrogen modes in α-FeOOH (Goethite) and γ-FeOOH (Lepidocrocite). J Phys Chem 99:5666–5673 Huang PM (2000) Abiotic catalysis. In: Summer ME (ed) Handbook of soil science. CRC, Boca Raton, Florida Huang PM (2004) Soil mineral–organic matter–microorganism interactions: fundamental and impacts. Adv Agron 82:391–472 Keppler F, Eiden R, Niedan V, Pracht J, Scholer HF (2000) Halocarbons produced by natural oxidation processes during degradation of organic matter. Nature 403:298–301 Kormann C, Bahnemann DW, Hoffmann MR (1989) Environmental photochemistry: is iron oxide (hematite) an active photocatalyst? A comparative study: α-Fe2O3, ZnO, TiO2. J Photochem Photobiol A Chem 48:161–169 Leland JK, Bard AJ (1987) Photochemistry of colloidal semiconducting iron oxide polymorphs. J Phys Chem 91:5076–5083 Mazellier P, Sulzberger B (2001) Diuron degradation in irradiated, heterogeneous iron/oxalate systems: the rate-determining step. Environ Sci Technol 35:3314–3320 Nadtochenko V, Kiwi J (1997) Photoinduced adduct formation between orange II and [Fe3+(aq)] or FeðoxÞ3 3 H2 O2 . J Chem Soc Faraday Trans 93:2373–2378 Pal B, Sharon M (1998) Photocatalytic degradation of salicylic acid by colloidal Fe2O3 particles. J Chem Technol Biotechnol 73:269–273 Rhoton FE, Bigham JM, Lindbo DL (2002) Properties of iron oxides in streams draining the loess uplands of Mississippi. Appl Geochem 17:409–419 Schoonen MAA, Xu Y, Strongin DRJ (1998) An introduction to geocatalysis. Geochem Exploration 62:201–215 Siffert C, Sulzberger B (1991) Light-induced dissolution of hematite in the presence of oxalate: a case study. Langmuir 7:1627–1634 Swearingen C, Macha S, Fitch A (2003) Leashed ferrocences at clay surfaces: potential applications for environmental catalysis. J Mol Catal A Chem 199:149–160 Voelker BM, Morel FMM, Sulzberger B (1997) Iron redox cycling in surface waters: effects of humic substances and light. Environ Sci Technol 31:1004–1011 Yu JG, Yu JC, Leung MKP, Ho WK, Cheng B, Zhao XJ, Zhao JC (2003) Effects of acidic and basic hydrolysis catalysts on the photocatalytic activity and microstructures of bimodal mesoporous titania. J Catal 217:69–78 Zhang Y, Charlet L, Schindler PW (1992) Adsorption of protons, Fe (II) and Al(III) on lepidocrocite (γ-FeOOH). Colloids Surf 63:259–268 Zuo YG, Holgné J (1992) Formation of hydrogen peroxide and depletion of oxalic acid in atmospheric water by photolysis of iron(III)-oxalato complexes. Environ Sci Technol 26: 1014–1022
